Python 3 becomes default for license compliance scanning

The License Compliance feature within the Secure stage will now default to using Python 3 for license scanning, aligning with the end-of-life for Python 2. Users on self-managed instances still using Python 2 can set the `LM_PYTHON_VERSION` CI variable to '2' to maintain compatibility. GitLab.com users will see this change automatically. This change is a proactive measure due to Python 2.7 reaching its end of life in January 2020.